The objective of this research is a quantitative analysis of the extracellular fields resulting from action potentials of individual neurons. The research uses cable theory, supplemented by a detailed kinetic description of the relevant voltage-dependent membrane conductances, combined with an efficient and accurate method for calculating the extracellular field from a 3-D distribution of current sources in a volume conductor. The model will be carefully compared with simultaneous intracellular and extracellular recordings of hippocampal neurons recorded in vivo. The first aim is to identify specific attributes of the fields that permits identification of the neuronal type, such as criteria that distinguish pyramidal neurons from inhibitory interneurons, neurons with backpropagating action potentials from those that don't, or recordings close to the apical dendrites from a recording close to the basal dendritic tree. The second aim is to determine the extracellular sampling bias of microelectrode recordings in the hippocampus by quantifying the effect on field amplitude of neuron size, shape and channel type distribution. This will be combined with the known distribution and density of the various hippocampal cell types, its geometry, and the bias caused by damage due to extracellular electrode insertion. Given the prevalence of extracellular recording techniques in systems neuroscience and, increasingly, in situations of clinical relevance (e.g. neuroprosthetics, single-unit recordings in epileptic patients), the proposed research should lead to a better understanding of what neuronal populations are preferentially recorded from.